The invention relates to a screw extruder in whose barrel, which accommodates at least one screw, bores are provided to accommodate pins which are arranged in radial planes and which reach into the barrel.
Such extruders have become known as pin-and-barrel extruders in a variety of embodiments, wherein the pins reach radially into the barrel and engage the gullets of the spiral screw. Such pin-and-barrel extruders are used mainly in single-screw extruders. It is precisely in these machines that the pins have acquired special importance in the plastification and mixing of the material to be extruded. They have been used in accordance with DE-A-01 37 813 for dough kneading and meat grinding machines. Pin extruders in accordance, for example, with U.S. Pat. No. 1,848,236, have been used in sausage manufacture. In the case of many other kinds of materials, such as those which have to be dewatered, special pin extruders have been disclosed, as in German Patent 1,454,801.
Pin extruders have acquired very great economic importance in the rubber industry through German Patent 22 35 784, since it was made possible by these machines to achieve high throughputs by means of cold-fed extruders (more than about 6000 kg/h). Cold feeding eliminates the need for large preheating machines for the formerly common hot-loaded extrusion, and has already been used world-wide in applications in the rubber industry which has to handle large throughputs, namely in the tire and conveyor belt industry. The problem has arisen that many of the cold-fed rubber mixtures, which are usually charged in the form of a continuous hide, were so hard or tough and require such great forces for plastification, that such radially disposed pins were subject to heavy wear, or in the most unfavorable case were even bent or broken. This has occasionally caused severe damage to the machinery. Attempts have of course been made to counteract this wear and this danger of breakage by appropriately designing the pins, and by improving the selection and treatment of the material. These efforts, however, are basically unsatisfactory, since at least after some amount of unavoidable pin wear the danger of breakage again arises, even when such danger did not exist when they were new. Pins with a shorter depth of penetration have also been used, or pins whose depth adjustment is variable, e.g., in accordance with DE-A-35 03 911 or DE-A-35 06 424. In that case, however, there is the disadvantage that a shorter working length or depth also results in lower effectiveness in plastification, which of course is undesirable.
Depth-adjustability also suffers from the possibility of error in critical cases by setting the pins too deep for the hardness of the composition to be plastified, and hence to cause damage to the pins.
Also, the complexity and cost of providing for the adjustment of the numerous pins are so high that only very little use has been made of it.
Pin-breakage alarm devices have also been disclosed, e.g., in German Patent Application 32 21 472, but they do not eliminate the danger of pin breakage, but rather increase it further because they weaken the pins, and they do no more than reduce the consequential damage, which they can hardly prevent. An improvement, if they are given the best possible configuration and are made of the best pin material, are the bent-pin alarms in accordance with German Patent Application 35 02 437, which permit the extruder to be shut down before appreciable damage has been done. These safety devices are relatively expensive, since a sensor is required for every endangered pin. Therefore, they are seldom used.
On account of the desired plastifying action of the pins, considerable friction heating is produced on the pins themselves in the extruding process, which manifests itself in the temperature rise in the particles of the mixture and in the pins. This heating, however, is ultimately disadvantageous and limits the maximum screw speed of the extruder, and hence also the extruder output and economy of operation.
A cooling of the pins already proposed in German Patent Application 22 35 784 has not yet been reduced to practice on account of the limited strength of the pins as described above.
One problem of pin extruders in the working of sticky compositions, such as many rubber mixtures which stick to the walls, consists in the adherence of mixture residues on the lee side of the pins--the so-called "dead corners." At least a partial fouling of the working chamber of the machine must in these cases be accepted when the machine is shut down. It is often cleaned up by running a cleaning mixture through it afterward, or when the extruder is restarted a certain initial fouling of the mixture must be tolerated, but both methods must be considered disadvantageous.
German Patent Application 38 05 849 provides for skewed pins which are intended to solve the problem of sticking mixtures and "dead corners," but this is possible only in an absolutely inadequate manner if the pins are not to be too greatly weakened. The same applies for similar reasons in the case of the streamlined pins according to German Patent Applications 36 13 584 and 36 13 612, which due to their configuration are suitable only for compositions that are not very difficult to work.
An invention covered by Italian patent 11 84 555 attempts to lessen the disadvantages and problems regarding strength by a nonradial arrangement, i.e., by setting the pins askew in the range of action of the extruder screw. The disadvantage of this construction is that the pins are made with a skewed or arcuate bottom surface and have to be mounted and secured at absolutely correct angles, which is not only expensive but also is hard to guarantee in the long run. The danger of breakage is reduced but not eliminated. Moreover, the problem of the "dead corners" is still worse than it is in the radial pin arrangement. The barrel cannot be made with a replaceable wear sleeve, and the available cooling surface area is less than in conventional extruders, so that throughput and economy are reduced. This type of construction has therefore not been accepted.